There is a method in which electrodes are formed at both ends of a magnetoresistance element that is an electromagnetic conversion element so as to form a Wheatstone bridge circuit, a constant-voltage power supply is connected between two opposing electrodes of this Wheatstone bridge circuit, and change in the resistance value of the magnetoresistance element is converted into voltage change, thereby detecting change in the magnetic field acting on the magnetoresistance element (see, for example, Patent document 1).
The magnetoresistance element here is, as shown in FIG. 7, provided with a laminate including a magnetization free layer 113 whose magnetization direction changes in response to an external magnetic field, a magnetization fixed layer 111 whose magnetization direction is fixed with respect to the external magnetic field, and a non-magnetic intermediate layer 112 that is sandwiched between the magnetization fixed layer 111 and the magnetization free layer 113. The magnetization of the magnetization free layer 113 freely rotates in response to the external magnetic field within a film plane of the laminate. Hereinafter, explanations will be made here with the magnetoresistance element exemplified by a tunnel magnetoresistance (hereinafter referred to as TMR) element whose non-magnetic intermediate layer 112 is made of an insulator.
The electric characteristics of the TMR element are generally known to be expressed in the form of a conductance G. (See Equation (2) and Column V: CONCLUSION, Non-patent document 1.)
That is, letting a relative angle of the magnetization direction of the magnetization free layer 113 with respect to that of the magnetization fixed layer 111 be θ, the conductance G can be expressed as below. Here, the magnetization direction of the magnetization free layer 113 becomes the same as the direction of the external magnetic field, that is, the rotation angle θ of the magnetic field.G=G0+G1 cos θ.  (1)
If this is expressed in the form of a resistance value, it becomes the inverse number of Equation (1) as follows:R=1/(G0+G1 cos θ).  (2)
FIG. 8 shows how the conductance G changes in response to the direction of the magnetic field applied to the TMR element from outside. In FIG. 8, the horizontal axis represents the rotation angle of the magnetic field, and the vertical axis represents the conductance G.
A conventional technology will be explained here, in which a half bridge (hereinafter referred to a bridge) is configured, as shown in FIG. 9, with TMR element connection bodies 116 and 117 each using eight TMR elements connected with each other, this bridge is disposed in front of a magnet body 114 that is alternately magnetized to N poles and S poles, and a voltage at the midpoint of the bridge is applied to an amplifier 119.
In FIG. 9, when the magnet body 114 moves in the left direction on this paper, magnetization directions of the magnetization fixed layers of all the TMR elements are those indicated by arrows 118. The direction 115 of the external magnetic field changes as shown in FIG. 8 depending on the position; therefore, the conductance G of the TMR element connection bodies 116 and 117 changes in the form of a cosine wave.
Here, the conductance G of TMR element connection body 116 and that of the TMR element connection body 117 are out of phase with each other by 180°. At this moment, the voltage at the midpoint of the bridge, which is a connecting point between the TMR element connection body 116 and the TMR element connection body 117, can be calculated using above-described Equation (2), which is given by the following Equation (3).(G0+G1 cos θ)/2G0.  (3)
Change in the voltage at the midpoint becomes in the form of a cosine wave, and an output waveform at an output terminal 120 becomes a cosine wave as shown by a curve 121, with the voltage at the midpoint inversely amplified by an amplifier 119. In this way, the change in the magnetic field is converted into voltage change, and thereby the movement of the magnet body, which is an object to be detected, can be detected.
There have been proposed various types of failure detection means in general-use sensing devices. For example, there is a device that detects change in voltage by applying a constant current to the midpoint of a bridge. (See, for example, Patent document 2.)
Moreover, there is another device in which switches are provided in parts of a bridge so as to monitor the resistance of the bridge by changing over those switches. (See, for example, Patent document 3.)